.png to .svg within blender
Directly from Blender only using python can be done via pypotrace. This is a python package that binds to potrace. potrace is also used by InkScape for converting images to Paths. You will need to include potrace and pypotrace in your add-on.

The result of the trace is a path. You will need to develop to you likings how to convert this to an SVG. But as SVG is just an XML file that will not be that hard.